Preheat oven to 400° F and line a baking sheet with parchment.
Toss cubed sweet potatoes with olive oil, cumin, salt, and pepper in a bowl. Arrange evenly on the baking sheet and roast for 20 to 25 minutes.
While the sweet potatoes are baking, simmer water, vinegar, salt, and sugar in a saucepan. Place red onions in a glass jar and pour in the vinegar mixture. Let cool and then refrigerate.
Blend avocado, sour cream, garlic, and lime juice until smooth.
Assemble tacos by spreading the avocado sauce over the tortilla and then topping with sweet potatoes, red beans, pickled onions, and parsley. Enjoy!